in sculpt mode the circle disappears when brushing
As of July 2020, there does not seem to be an option in Blender to display the sculpting circle cursor during a stroke.
This seems like a huge oversight and workflow regression in my opinion. Sculpting with pressure-dependent radius feels like I'm trying to paint using an invisible brush.
However, there is a code contribution on developer.blender.org that adds this functionality back in:
⚙ D6324 Sculpt: Sculpt plane cursor preview (https://developer.blender.org/D6324)
It appears to be in need of work. They want some code changes, as well as explanations of why it's a positive change.
If you'd like to see this feature added, and you know how to code or just have time to spare, maybe consider leaving a thoughtful comment or helping with the maintenance on the linked code.

As a workaround, I've been enabling &quot;Falloff Opacity&quot; under &quot;Cursor&quot; in the brush settings. Running this line in a Python Console view will do it for all brushes: [[setattr(b, 'use_cursor_overlay', True), setattr(b, 'cursor_overlay_alpha', 5)] for b in bpy.data.brushes.values()].